Process for producing poly (para-phenylene-sulfide)

ABSTRACT

In a process for producing poly (p-phenylenesulfide) from an alkali metal sulfide and a p-dihalobenzene, a product polymer of an advantageously increased molecular weight is obtained by: providing a mixture comprising an alkali metal sulfide, a polar aprotic solvent and a carboxylic acid sodium salt represented by the general formula: ZCOONa (wherein Z is a C 6  -C 20  aromatic hydrocarbon group or a substituted or unsubstituted pyridyl group ##STR1## where substituents R, which may be the same or different, represent each a C 1  -C 20  organic radical and n is an integer of from 0 to 4), the content of water present in said mixture apart from any water of hydration or crystallization which may be contained in said alkali metal sulfide and carboxylic acid sodium salt being at least 5 moles per mole of said alkali metal sulfide present; thermally dehydrating said alkali metal sulfide in said mixture by removing at least part of the water from said mixture; and then contacting the resulting dehydrated mixture with a p-dihalobenzene so as to produce the intended polymer.

This is a continuation of application Ser. No. 07/288,929 filed Dec. 23,1988, now abandoned.

This invention relates to a process for producing poly(p-phenylenesulfide), more particularly, to a process for producing poly(p-phenylenesulfide) of increased molecular weight.

Poly (p-phenylenesulfide) has high resistance to heat and chemicals anduse of it in electrical and electronic parts, as well as in automotiveparts is drawing attention of researchers. This polymer can beinjection-molded, extrusion-molded or otherwise molded into variousshaped articles including films, sheets and fibers, and the resultingshaped articles find extensive use in applications where resistance toheat and chemicals is required.

One conventional method for producing poly (p-phenylenesulfide) consistsof reacting a dihalo aromatic compound with an alkali metal sulfide suchas sodium sulfide in a polar aprotic solvent (see Japanese PatentPublication No. 45-3368). However, the polymer produced by this methodis too low in molecular weight to be usable in molding applicationsincluding injection molding. In order for this low-molecular weightpolymer to be used in shaping and processing applications, it isconventionally crosslinked by thermal oxidation to increase itsmolecular weight. However, even this polymer having increased molecularweight has low adaptability for extrusion, probably due to the highdegree of crosslinking and branching and substantial difficulty isinvolved in shaping it into films or fibers.

In an attempt to solve this problem, methods have been proposed forobtaining poly (p-phenylenesulfide) of increased molecular weight bypolymerization reaction. A typical example of this approach is describedin Japanese Patent Publication No. 52-12240 and consists of performingthe intended polymerization reaction in the presence of R-COOM (R is ahydrocarbyl group and M is an alkali metal) which is used as apolymerization aid. Another method is described in our copendingJapanese Patent Application No. 61-303655, in which the polymerizationreaction is effected in the presence of a polymerization aid comprisingan alkali metal salt of substituted or unsubstituted pyridine carboxylicacid represented by the formula: ##STR2## wherein substituents Rrepresent each a C₁ -C₂₀ organic group, M is an alkali metal and n is aninteger of 0-4. The polymers of increased molecular weights obtained bythese methods have high adaptability for extrusion molding and caneffectively be formed into films, fibers and other shaped articles.

A problem with these methods, however, is that only expensive lithiumsalts used as polymerization aids will exhibit marked effectiveness inproviding increased molecular weight and hence, the production cost isincreased to a commercially unfeasible level. On the other hand,inexpensive sodium salts are inefficient to provide an intended increasein molecular weight unless a crosslinking agent such as a polyhaloaromatic compound containing at least 3 halogens in one molecule isadded. However, not only does this intricate the operations inproduction process but also the resulting polymer is prone to gelation.

An object, therefore, of the present invention is to provide a processfor producing poly (p-phenylenesulfide) of increased molecular weight byusing a sodium salt of carboxylic acid that is at least comparable tolithium salts in its ability to increase the molecular weight of thepolymer.

This object of the present invention can be attained by a process forproducing poly (p-phenylenesulfide) which comprises:

providing a mixture comprising an alkali metal sulfide, a polar aproticsolvent, and a carboxylic acid sodium salt represented by the generalformula: ZCOONa (where Z is a C₆ -C₂₀ aromatic hydrocarbon group or asubstituted or unsubstituted pyridyl group ##STR3## where substituentsR, which may be the same or different, represent each a C₁ -C₂₀ organicradical and n is an integer of from 0 to 4), the content of waterpresent in said mixture apart from any water of hydration orcrystallization which may be contained in said alkali metal sulfide andcarboxylic acid sodium salt being at least 5 moles per mole of saidalkali metal sulfide present;

thermally dehydrating said alkali metal sulfide in said mixture byremoving at least part of the water from said mixture; and

then contacting the resulting dehydrated mixture with a p-dihalobenzeneso as to produce the intended polymer.

Generally, said at least 5 moles of water per mole of said sulfide isadded to the mixture prior to the thermally dehydrating stage.

Examples of the carboxylic acid sodium salts (where Z is a C₆ -C₂₀aromatic hydrocarbon group) for use in the invention include sodiumsalts of benzoic, o-toluic, m-toluic, p-toluic, o-ethylbenzoic, cuminic,4-N-propylbenzoic, 2,3,4-trimethylbenzoic, 2,3,4,5-tetramethylbenzoic,pentamethylbenzoic, 1-naphthoic, 2-naphthoic, anthracene-1-carboxylic,anthracene-2-carboxylic, anthracene-9-carboxylic,4-phenylanthracene-1-carboxylic, phenanthrene-1-carboxylic,phenanthrene-2-carboxylic and phenanthrene-9-carboxylic acids, andmixtures of these salts.

Examples of the carboxylic acid sodium salts (where Z is a substitutedor unsubstituted pyridyl group) for use in the invention include sodiumsalts of nicotinic, 2-methylnicotinic, 4-methylnicotinic,5-methylnicotinic, 6-methylnicotinic; 2,4-dimethylnicotinic;2,5-dimethylnicotinic; 2,6-dimethylnicotinic; 4,6-dimethylnicotinic; and5,6-dimethylnicotinic acids, and mixtures of these salts.

The sodium salts of carboxylic acids for use in the present inventionmay be either anhydrous or hydrous. These salts are added in amountsthat generally range from 0.05 to 4 moles, preferably from 0.1 to 2moles, per mole of p-dihalobenzene. If the addition of the sodium saltof carboxylic acid is less than 0.05 moles per mole of p-dihalobenzene,the salt is not sufficiently effective to increase the molecular weightof the end polymer.

If the addition of the sodium salt of carboxylic acid is more than 4moles per mole of p-dihalobenzene, troubles will occur such asdifficulty in agitating the contents of the reactor. It is essential forthe purpose of the present invention that the sodium salt of carboxylicacid should be present in the reaction system before the alkali metalsulfide is dehydrated.

In the method of the present invention, water is added to the reactionsystem before dehydration of the alkali metal sulfide and the water tobe added should be free water which is clearly distinguished from thewater of crystallization in alkali metal sulfides or salts of carboxylicacids. It is important for the purpose of the present invention thatsuch "free water" should be present in an amount of at least 5 moles permole of the alkali metal sulfide in the reaction system beforedehydration. If such water is present in an amount of less than 5 molesper mole of the alkali metal sulfide, it is insufficient to enhance ofability of the sodium salt of carboxylic acid to increase the molecularweight of the end polymer. If a substantially large amount of freewater, such as more than about 40 or 50 moles of free water is presentper mole of the alkali metal sulfide, more energy is required to achievedehydration of the alkali metal sulfide, which is disadvantageous froman economic viewpoint.

In order to enhance the ability of the sodium salt of carboxylic acid toincrease the molecular weight of the end polymer, the alkali metalsulfide, polar aprotic solvent, sodium salt of carboxylic acid and theabove-specified amount of water must be present simultaneously in thereaction system prior to dehydration of the alkali metal sulfide. Theobject of the present invention is achieved only when the alkali metalsulfide is dehydrated after these four components are incorporated inthe reaction system.

Exemplary alkali metal sulfides for use in the present invention includelithium sulfide, sodium sulfide, potassium sulfide, rubidium sulfide,cesium sulfide and mixtures thereof. These sulfides or mixtures thereofmay be used in the form of a hydrate. These alkali metal sulfides areprepared by reacting alkali metal hydrosulfides with alkali metal bases,or hydrogen sulfide with alkali metal bases. They may be prepared eitherin situ or outside of the reaction system before they are added to thesystem for polymerization of p-dihalobenzene. Among the alkali metalsulfides listed above, sodium sulfide is particularly preferred for usein the present invention.

Before adding p-dihalobenzene for polymerization, water is preferablyremoved from the reaction system by distillation or some other suitablemethod so that its content will be no more than about 4 moles per moleof the alkali metal sulfide. It is also possible to adjust the amount ofwater in the reaction system during the process of polymerization.

Illustrative p-dihalobenzenes that can be used in the present inventioninclude p-dichlorobenzene, p-dibromobenzene, p-diiodobenzene andmixtures thereof, with p-dichlorobenzene being particularlyadvantageous. Other monomers may be copolymerized with p-dihalobenzeneas long as their content is less than 30 mol % of p-dihalobenzene, andexamples of such copolymerizable monomers include m-dihalobenzenes suchas m-dichlorobenzene, o-dihalobenzenes such as o-dichlorobenzene, anddihalo aromatic compounds such as dichloronaphthalene,dibromonaphthalene, dichlorodiphenylsulfone, dichlorobenzophenone,dichlorodiphenylether, dichlorodiphenyl sulfide, dichlorodiphenyl,dibromodiphenyl and dichlorodiphenyl sulfoxide. If desired, polyhaloaromatic compounds containing at least 3 halogens in one molecule mayalso be employed in small enough amounts not to impair the linearity ofthe end polymer, and illustrative polyhalo aromatic compounds includetrichlorobenzene, tribromobenzene, triiodobenzene, tetrachlorobenzene,trichloronaphthalene and tetrachloronaphthalene.

Polar solvents are preferably used as solvents for polymerization in theprocess of the present invention, and particularly preferred solventsare those which are aprotic and which are stable against alkalies atelevated temperatures. Exemplary solvents for polymerization includeN,N-dimethylacetamide, N,N-dimethylformamide, hexamethylphosphorylamide, N-methyl-ε-caprolactam, N-ethyl-2-pyrrolidone,N-methyl-2-pyrrolidone, 1,3-dimethylimidazolidinone, dimethyl sulfoxide,sulfolane, tetramethylurea, and mixtures thereof.

Polymerization of p-dihalobenzene is performed at 200°-300° C.,preferably at 220°-280° C., for a period of 0.5-30 hours, preferably1-15 hours, under agitation. The molar ratio of the alkali metal sulfideto p-dihalobenzene which are to be used in the process of the presentinvention is preferably in the range of from 1:0.9 to 1:1.1. Polaraprotic solvents may be used in such amounts that 3-60 wt %, preferably7-40 wt %, of the polymer will be present in the mixture afterpolymerization.

The resulting poly (p-phenylenesulfide) may be recovered from thereaction mixture by any ordinary method, such as a method consisting ofremoving the solvent by distillation, flashing or some other suitablemeans, washing the polymer with water, and recovering it, or a methodconsisting of removing the solvent by filtering the reaction mixture,washing the polymer with water and recovering it. The second method ispreferred since it imparts a minimum degree of thermal history to thepolymer, thereby preventing coloration or gelation of the polymer.

The poly (p-phenylenesulfide) which is the end product of the process ofthe present invention must contain at least 70 mol % of ##STR4## as theconstituent units. It may also contain copolymerizable units if theircontent is less than 30 mol % of the polymer, and examples of suchcopolymerizable units include: m-phenylene sulfide unit ##STR5##

The poly (p-phenylenesulfide) thus produced by the process of thepresent invention has its molecular weight increased in a linear form,so it is suitable for use as a material that is to be extrusion-moldedinto shaped articles such as fibers, films and tubes. If necessary,various additives may be incorporated in the polymer and illustrativeadditives include: ceramic fibers such as glass fibers, carbon fibersand alumina fibers; reinforcing fillers such as aramid fibers, totallyaromatic polyester fibers, metal fibers and potassium titanate whiskers;inorganic fillers such as calcium carbonate, mica, talc, silica, bariumsulfate, calcium sulfate, kaolin, clay, pyroferrite, bentonite,sericite, zeolite, nepheline syenite, attapulgite, wollastonite,ferrite, calcium silicate, magnesium carbonate, dolomite, antimonytrioxide, zinc oxide, titanium oxide, magnesium oxide, iron oxide,molybdenum disulfide, graphite, gypsum, glass beads, glass powder, glassbaloons, quartz, and silica glass; and organic or inorganic pigments.

Other additives that may be incorporated as required includeplasticizers, mold release agents such as aromatic hydroxy derivatives,silane or titanate based coupling agents, lubricants, heat stabilizers,weather-proofing agents, nucleating agents, foaming agents, corrosioninhibitors, ion trapping agents, flame retardants and flame retardingaids.

If necessary, homopolymers, random copolymers, block copolymers andgraft copolymers, either on their own or as admixtures, may be mixedwith the poly (p-phenylenesulfide), and they include: polyethylene;polybutadiene; polyisoprene; polychloroprene; polystyrene; polybutene;poly-α-methylstyrene; polyvinyl acetate; polyvinyl chloride;polyacrylate esters; polymethacrylate esters; polyacrylonitrile;polyamides such as nylon 6, nylon 66, nylon 610, nylon 12, nylon 11, andnylon 46; polyesters such as polyethylene terephthalate, polybutyleneterephthalate and polyarylate; polyurethane; polyacetal; polycarbonate;polyphenylene oxide; polysulfone; polyether sulfone; polyaryl sulfone;polyether ketone; polyether ether ketone; polyimide; polyamideimide;silicone resins; phenoxy resins; and fluorine resins.

The following examples are provided for the purpose of furtherillustrating the present invention but are in no way to be taken aslimiting.

In the examples and comparative examples shown below, the meltviscosities of the samples of poly (p-phenylenesulfide) prepared weremeasured with a KOHKA-type flow tester (die, 0.5 mm.sup.φ ×2 mm^(L)) at300° C. under a load of 10 kg.

EXAMPLE 1

A 500-ml autoclave was charged with 0.5 moles of sodium sulfide Na₂S.2.9H₂ O, 125 ml of N-methyl-2-pyrrolidone (hereinafter abbreviated asNMP), 0.15 moles of sodium benzoate and 12.85 moles of distilled water.The temperature in the autoclave was raised to 200° C. with stirringunder a nitrogen stream so as to distill off 247.0 g of wateraccompanied with 15.8 g of NMP. After cooling the system to 170° C., 0.5moles of p-dichlorobenzene (hereinafter abbreviated as p-DCB) was addedtogether with 42 ml of NMP and the system was closed under a nitrogenstream, followed by polymerization at 245° C. for 10 hours. Aftercompletion of the polymerization, the system was cooled and the contentswere thrown into water. Following repeated cycles of washing with about5 l of warm water and filtration, the residual cake was washed once withmethanol and vacuum-dried with heating overnight to obtain small whitegranules of poly (p-phenylenesulfide). The polymer yield was 93 % andits melt viscosity was 190 Pa.s (see Table 1).

EXAMPLES 2-9

Polymerization was performed as in Example 1 except that the molar ratioof Na₂ S to p-DCB charged, the amount of water added (molar ratio ofadded water to Na₂ S), the type of the sodium salt of aromaticcarboxylic acid, the amount of its addition (molar ratio of ZCOONa toNa₂ S), the polymerization temperature and time were changed as shown inTable 1. The results are also shown in Table 1.

COMPARATIVE EXAMPLE 1

Polymerization was performed as in Example 1 except that water was notadded and that the molar ratio of Na₂ S to p-DCB was 0.98. The resultingpolymer was in the form of small granules; its yield was 94% and it hada melt viscosity of 73 Pa.s. This result shows that sodium benzoate usedin the absence of water was not highly effective in increasing themolecular weight of the end polymer (see Table 1).

COMPARATIVE EXAMPLE 2

Polymerization was performed as in Example 1 except that water was addedin an amount of 3 moles per mole of sodium sulfide. The resultingpolymer was in the form of small granules; its yield was 93% and it hada melt viscosity of 90 Pa.s. This result shows that the ability ofsodium sulfide to increase the molecular weight of the end polymer wasnot satisfactorily high when the amount of water added was less than 5moles per mole of sodium sulfide (see Table 1).

COMPARATIVE EXAMPLE 3

Polymerization was performed as in Example 1 except that Na₂ S.2.9H₂ Owas replaced by Na₂ S.9H₂ O and no water was added. The resultingpolymer was in the form of small granules; its yield was 92% and it hada melt viscosity of 100 Pa.s. This result shows that the water ofcrystallization present in the polymerization system did not contributemuch to the increase in the molecular weight of the end polymer (seeTable 1).

COMPARATIVE EXAMPLE 4

Polymerization was performed as in Example 1 except that a mixture ofsodium benzoate, water and NMP (containing no sodium sulfide) was heatedat 200° C. to achieve partial dehydration and that thereafter, sodiumsulfide was added to the system which was re-heated at 200° C. toachieve dehydration. The resulting polymer was in the form of smallgranules; its yield was 92% and it had a melt viscosity of 114 Pa.s,which was lower than the values attained in Examples 1-8 (see Table 1).

COMPARATIVE EXAMPLE 5

Polymerization was performed as in Example 1 except that the sodiumbenzoate was introduced together with p-DCB and NMP into the systemafter completion of dehydration. The resulting polymer was in the formof small granules; its yield was 95% and it had a melt viscosity of 79Pa.s, which was lower than the values attained in Examples 1-9 (seeTable 1).

As Comparative Examples 1-5 show, the sodium salt of aromatic carboxylicacid is not highly effective in increasing the molecular weight of theend polymer if it is not present together with NMP, sodium sulfide and aspecified amount of water in the reaction mixture to be dehydrated. Theadvantages of the present invention can be achieved only when themixture containing the above-specified components is dehydrated.

                                      TABLE 1                                     __________________________________________________________________________                             H.sub.2 O/Na.sub.2 S                                                                       ZCOONa/                                                                             Polymer-                                                                           Polymer- Melt                           Na.sub.2 S/                                                                          Added  molar ratio  Na.sub.2 S                                                                          ization                                                                            ization  vis-                           p-DCB  Water/Na.sub.2 S                                                                     after        (molar                                                                              tempera-                                                                           time Yield                                                                             cosity              Na.sub.2 S (molar ratio)                                                                        (molar ratio)                                                                        dehydration                                                                         ZCOONa ratio)                                                                              ture (°C.)                                                                  (hr) (%) (Pa.s)              __________________________________________________________________________    Ex. 1                                                                             Na.sub.2 S.2.9H.sub.2 O                                                              1.00   25.7   1.1   sodium 0.30  245  10   93  190                                                benzoate                                                                      (anhydrous)                                    Ex. 2                                                                             "      1.02   25.6   "     sodium "     "     5   94  172                                                benzoate                                                                      (anhydrous)                                    Ex. 3                                                                             "      "      "      1.0   sodium "     "    10   92  208                                                benzoate                                                                      (anhydrous)                                    Ex. 4                                                                             "      1.00   "      1.1   sodium "     "    20   "   177                                                benzoate                                                                      (anhydrous)                                    Ex. 5                                                                             "      "      28.9   1.8   sodium 0.50  "    10   93  197                                                benzoate                                                                      (anhydrous)                                    Ex. 6                                                                             "      "      36.0   2.4   sodium 1.00  "    "    95  152                                                benzoate                                                                      (anhydrous)                                    Ex. 7                                                                             "      "      25.6   1.4   sodium 0.30  250   5   94  147                                                benzoate                                                                      (anhydrous)                                    Ex. 8                                                                             "      "      "      1.3   sodium "     "    10   93  176                                                benzoate                                                                      (anhydrous)                                    Ex. 9                                                                             "      "      32.3   1.2   sodium m-                                                                            "     245  "    94  167                                                toluylate                                                                     (anhydrous)                                    Comp.                                                                             "      0.98   0      1.1   sodium "     "    "    "    73                 Ex. 1                          benzoate                                                                      (anhydrous)                                    Comp.                                                                             "      1.00    3.0   1.2   sodium "     "    "    93   90                 Ex. 2                          benzoate                                                                      (anhydrous)                                    Comp.                                                                             Na.sub. 2 S.9H.sub.2 O                                                               "      0      "     sodium "     "    "    92  100                 Ex. 3                          benzoate                                                                      (anhydrous)                                    Comp.                                                                             Na.sub.2 S.2.9H.sub.2 O                                                              "      15.7   1.4   sodium "     "    "    "   114                 Ex. 4*.sup.1                   benzoate                                                                      (anhydrous)                                    Comp.                                                                             "      "      25.6   1.1   sodium "     "    "    95   79                 Ex. 5*.sup.2                   benzoate                                                                      (anhydrous)                                    __________________________________________________________________________     *.sup.1 A mixture of sodium benzoate, H.sub.2 O and NMP was dehydrated by     heating, followed by addition of Na.sub.2 S and reheating for dehydration     *.sup.2 After dehydration of Na.sub.2 S, sodium benzoate was added            together with pDCB and NMP.                                              

EXAMPLE 10

A 500-ml autoclave was charged with 0.5 moles of sodium sulfide Na₂S.2.9H₂ O, 125 ml of NMP, 0.15 moles of sodium nicotinate and 9.5 molesof distilled water. The temperature in the autoclave was raised to 200°C. with stirring under a nitrogen stream so as to distill off 186.2 g ofwater accompanied with 11.9 g of NMP. After cooling the system to 170°C., 0.50 moles of p-DCB was added together with 42 ml of NMP and thesystem was closed under a nitrogen stream, followed by polymerization at245° C. for 10 hours. After completion of the polymerization, the systemwas cooled and the contents were thrown into water. Following repeatedcycles of washing with about 5 l of warm water and filtration, theresidual cake was washed once with methanol and vacuum-dried withheating overnight to obtain small white granules of poly(p-phenylenesulfide). The polymer yield was 93% and its melt viscositywas 211 Pa.s (see Table 2).

EXAMPLES 11-14

Polymerization was performed as in Example 1 except that the amount ofwater added (molar ratio of added water to Na₂ S), the type of thesodium salt of carboxylic acid, the amount of its addition (molar ratioof ZCOONa to Na₂ S), the polymerization temperature and time werechanged as shown in Table 2. The results are also shown in Table 2.

COMPARATIVE EXAMPLE 6

Polymerization was performed as in Example 10 except that water was notadded and that polymerization consisted of two stages, the first stagebeing continued for 2 hours at 230° C. and the second stage continuedfor 2 hours at 265° C. The resulting polymer was in the form ofgranules; its yield was 96% and it had a melt viscosity of 110 Pa.s.This result shows that sodium nicotinate used in the absence of waterwas not highly effective in increasing the molecular weight of the endpolymer (see Table 2).

COMPARATIVE EXAMPLE 7

Polymerization was performed as in Example 10 except that water wasadded in an amount of 3 moles per mole of sodium sulfide. The resultingpolymer was in the form of small granules; its yield was 94% and it hada melt viscosity of 119 Pa.s. This result shows that the ability ofsodium sulfide to increase the molecular weight of the end polymer wasnot satisfactorily high when the amount of water added was less than 5moles per mole of sodium sulfide (see Table 2).

COMPARATIVE EXAMPLE 8

Polymerization was performed as in Example 10 except that Na₂ S.2.9H₂ Owas replaced by Na₂ S.9H₂ O, and no water was added and that thereaction time was 5 hours. The resulting polymer was in the form ofsmall granules; its yield was 92% and it had a melt viscosity of 88Pa.s. This result shows that the water of crystallization present in thepolymerization system did not contribute much to the increase in themolecular weight of the end polymer (see Table 2).

COMPARATIVE EXAMPLE 9

Polymerization was performed as in Example 10 except that a mixture ofsodium sulfide, sodium nicotinate and water was heated at 200° C. toachieve partial dehydration and that thereafter, NMP was added to thesystem which was re-heated at 200° C. to achieve dehydration. Theresulting polymer was in powder form; its yield was 94% and it had amelt viscosity of 30 Pa.s, which was lower than the values attained inExamples 10-14 (see Table 2).

COMPARATIVE EXAMPLE 10

Polymerization was performed as in Example 1 except that a mixture oftwo moles of water per mole of sodium sulfide to be used and sodiumnicotinate was dehydrated by heating at 200° C. in NMP, and thatthereafter, sodium sulfide and three moles of water per mole of sodiumsulfide were added to the mixture, which was re-heated at 200° C. toachieve dehydration. The resulting polymer was in the form of smallgranules; its yield was 94% and it had a melt viscosity of 97 Pa.s,which was lower than the values attained in Examples 10-14 (see Table2).

COMPARATIVE EXAMPLE 11

Polymerization was performed as in Example 1 except that sodiumnicotinate was added together with p-DCB and NMP after completion ofdehydration. The resulting polymer was in the form of small granules;its yield was 92% and it had a melt viscosity of 85 Pa.s, which waslower than the values attained in Examples 10-14 (see Table 2).

As Comparative Examples 6-11 show, the sodium salt of pyridinecarboxylicacid is not highly effective in increasing the molecular weight of theend polymer if it is not present together with NMP, sodium sulfide and aspecified amount of water in the reaction mixture to be dehydrated. Theadvantages of the present invention can be achieved only when themixture containing the above-specified components is dehydrated.

                                      TABLE 2                                     __________________________________________________________________________    Added       H.sub.2 O/Na.sub.2 S                                                                   Sodium                                                                              Sodium pyridine-                                                                       Polymerization        Melt                water/Na.sub.2 S                                                                          (molar ratio)                                                                          pyridine-                                                                           carboxylate/Na.sub.2 S                                                                 tempera-                                                                              Polymerization                                                                              viscosity           (molar ratio)                                                                             after dehydration                                                                      carboxylate                                                                         (molar ratio)                                                                          ture (°C.)                                                                     time (hr)                                                                             Yield                                                                               (Pa.s)              __________________________________________________________________________    Ex. 10                                                                             19     1.2      sodium                                                                              0.30     245     10      93    211                                      nicotinate                                               Ex. 11                                                                             16     1.3      sodium                                                                              "        250     "       94    185                                      nicotinate                                               Ex. 12                                                                             11     1.1      sodium                                                                              "        245     "       92    180                                      nicotinate                                               Ex. 13                                                                             19     1.0      sodium                                                                              0.50     250     5       93    194                                      nicotinate                                               Ex. 14                                                                             "      1.3      6-methyl                                                                            0.30     245     10      92    176                                      sodium                                                                        nicotinate                                               Comp.                                                                               0     1.1      sodium                                                                              "        230     2       96    110                 Ex. 6                nicotinate     265     2                                 Comp.                                                                               3     1.0      sodium                                                                              "        245     10      94    119                 Ex. 7                nicotinate                                               Comp.                                                                               0     1.3      sodium                                                                              "        "       5       92     88                 Ex. 8*               nicotinate                                               Comp.                                                                              19     0.5      sodium                                                                              "        "       10      94     30                 Ex. 9*.sup.2         nicotinate                                               Comp.                                                                              .sup.  1.2.sup.3                                                                              sodium                                                                              "        "       5       "      97                 Ex. 10               nicotinate                                               Comp.                                                                              19     1.3      sodium                                                                              "        "       "       92     85                 Ex. 11*.sup.4        nicotinate                                               __________________________________________________________________________     *.sup.1 Na.sub.2 S.9H.sub.2 O was used.                                       *.sup.2 A mixture of Na.sub.2 S, sodium nicotinate and H.sub.2 O was          dehydrated by heating, followed by addition of NMP and reheating for          dehydration.                                                                  *.sup.3 A mixture of sodium nicotinate and 2 moles of H.sub.2 O per mole      of Na.sub.2 S to be added was dehydrated by heating in NMP, and               thereafter, Na.sub.2 S and 3 moles of H.sub.2 O per mole of Na.sub.2 S        were added to the mixture, which was reheated for dehydration.                *.sup.4 After dehydration of Na.sub.2 S, sodium nicotinate was added          together with pDCB and NMP.                                              

As will be understood from the foregoing description, the process of thepresent invention allows a poly (p-phenylenesulfide) of increasedmolecular weight to be produced even if a sodium salt of carboxylic acidis used as a polymerization aid. The resulting poly (p-phenylenesulfide)is suitable for use not only in injection-molding applications but alsoin producing films, fibers and other shaped articles by extrusionmolding.

What is claimed is:
 1. A process for producing poly (p-phenylenesulfide)which comprises:providing a mixture comprising an alkali metal sulfide,a polar aprotic solvent and a carboxylic acid sodium salt represented bythe general formula: ZCOONa (wherein Z is a C₆ -C₂₀ aromatic hydrocarbongroup), the content of water present in said mixture apart from anywater of hydration or crystallization which may be contained in saidalkali metal sulfide and carboxylic acid sodium salt being at leastabout 25 moles per mole of said alkali metal sulfide present; thermallydehydrating said alkali metal sulfide in said mixture by removing atleast part of the water from said mixture wherein about 1 to about 2.4mols of water per mol of Na₂ S remains after dehydration; and thencontacting the resulting dehydrated mixture with a p-dihalobenzene so asto produce the intended polymer.
 2. A process according to claim 1wherein the proportion of the water is up to about 50 moles per mole ofsaid sulfide.
 3. A process according to claim 1 wherein the carboxylicacid sodium salt is selected from the group consisting of sodium saltsof benzoic, o-toluic, m-toluic, p-toluic, o-ethylbenzoic, cuminic,4-N-propylbenzoic, 2,3,4-trimethylbenzoic, 2,3,4,5-teramethylbenzoic,pentamethylbenzoic, 1-naphthoic, 2-naphthoic, anthracene-1-carboxylic,anthracene-2-carboxylic, anthracene-9-carboxylic,4-phenylanthracene-1-carboxylic, phenanthrene-1-carboxylic,phenanthrene-2-carboxylic and phenanthrene-9-carboxylic acids, andmixtures of these salts.
 4. A process according to claim 1 wherein thealkali metal sulfide is selected from the group consisting of sulfidesof lithium, sodium, potassium, rubidium and cesium and mixtures of thelisted sulfides.
 5. A process according to claim 1 wherein thep-dihalobenzene is selected from the group consisting of p-dichloro-,p-dibromo- and p-diiodo-benzenes and mixtures thereof.
 6. A processaccording to claim 5 wherein the p-dihalobenzene is employed incombination with one or more other dihaloaromatic compounds that arepresent in a proportion of less than 30 mol % of the p-dihalobenzene. 7.A process according to claim 1 wherein the solvent is selected from thegroup consisting of N,N-dimethylacetamide, N,N-dimethylformamide,hexamethyl phosphorylamide, N-methyl-ε-caprolactam,N-ethyl-2-pyrrolidone, N-methyl-2-pyrrolidone,1,3-dimethylimidazolidinone, dimethyl sulfoxide, sulfolane,tetramethylurea, and mixtures thereof.
 8. A process according to claim 1wherein the dehydrated mixture is contacted with the p-dihalobenzene ata temperature of about 200°-300° C. for a period of 0.5-30 hours withstirring.
 9. A process according to claim 1 wherein the molar ratio ofthe alkali metal sulfide to p-dihalobenzene used is in the range of from1.00:0.90 to 1.00:1.10.
 10. A process according to claim 1 wherein thepolar aprotic solvent is employed in such an amount that 3-60% by weightof the polymer is present in the mixture after polymerization.